Hi John
You appear to be able to construct your postings in a clear and concise way, so I'm a little confussed as to why you say that you struggle with achieving qualifications??
On a personal level, l started straight from school, at a small rural family owned garage, undertaking anything which came through the door. I also did my day release apprenticeship. However at the age of 21 and passed with flying colours I thought I knew it all and pushed for top line pay. - I had my P45.
I then started my own workshop and realised how little I knew about the workings of cars, as I had nobody to turn to and the ilexa forum wasn't about in those days

As such, I agree with the others when they say that you have got to know your subject inside out and how it is supposed to work before you complicate matters with trying to find out why a car is poorly. But as you say, how do you get that experience??
Every day is a school day for me and I have new challenges to get my head around and as technology progresses I'm always playing catchup. Although it's always interesting, the amount of none chargeable time I spend each day learning and doing research is frightening.
If you haven't been put off by the comments by now, the one area which is so critical to today's vehicles is electrical components. You must be able to understand electrical circuits. Without a clear understanding of this area, you are stumped and I mean a clear understanding, not just be able to recall a formula, but know why it does what it does and if it behaves in a strange way, know why and where to start looking.
This would be the one area I would concentrate on.
Get knowledge in this area and the smaller garages, who have previously resisted change and now find that they open a bonnet of a modern car and then close it again, because they haven't a clue, will treat you like a messiah.
Will H
PS
.......................Or get your head around keys, locks and immobilizers - a head scratcher to most.